1. Field of the Invention
The present invention relates to amplifiers and more particularly to switched-mode (class-D) power amplifiers.
2. Related Art
Typically, switched-mode (class-D) power amplifiers offer very high efficiencies at large levels of sinusoidal or other artificial test signals. However, when applied to music signals, the levels of which are low in the average, idle power loss becomes a significant factor. A typical 100 W class-D amplifier may dissipate 3 W or more of power at low levels. In applications where a large number of amplifiers (5-10) are required in small spaces, such as audio/video receivers or soundbars, total power dissipation becomes a major issue. Idle power loss may be minimized, but traditionally at the expense of introducing distortion around zero crossing of the signal (crossover distortion). This form of distortion leads to information loss, is particularly audible, and is not correctly represented by traditional distortion measures such as THD (total harmonic distortion).
Crossover distortion in conventional class-D amplifiers, which use a pair of n-channel Mosfet transistors as power switches, is caused by “dead-time.” A certain amount of time, typically (20-100) nsec, must be inserted during switching transitions in order to prevent both switches being turned on at the same time, which if turned on at the same time, would cause very high current peaks (“shoot-through”).
Switching without dead-time may be possible with a pair of complementary Mosfets in source follower configuration (“totem pole”). In this configuration, the gates and sources of both devices are tied together, respectively. The gate voltage of either device has to decrease below its own threshold before it can exceed the threshold of the other device. It is inherently impossible to turn both devices on simultaneously with this configuration.
The main problem with implementing the source follower totem pole is the gate drive circuit. This is because the drive voltage must exceed the power supply rail voltage by at least the source-gate threshold voltage to turn either device on. In the past, the circuits to generate this threshold voltage have been complicated and expensive relative to the cost of the rest of the circuit.
Turning to FIG. 1, a prior art circuit 100 is shown where a pulse transformer is used to drive a source-follower totem pole. A problem with this circuit is the cost and size of the transformer, and its limited bandwidth, in particular low-frequency extension. In FIG. 2, another prior art circuit is shown that discloses a source follower gate drive circuit 200 and may be seen in U.S. Pat. No. 6,856,193. This approach requires a so-called “differential comparator,” whose supply voltage must be referenced to the amplifier output, rather than ground, and a floating supply voltage V1, see FIG. 2. Further details are not given in this patent application and floating power supplies are expensive, difficult to implement, and may cause major electromagnetic interference problems that require even more expense to isolate or eliminate.
Accordingly, a need exists for a simple, low cost circuit that generates the necessary drive voltage for use in a source follower totem pole gate drive circuit. In particular, it is desirable to have a low cost gate drive circuit that overcomes the limitations and problems described above.